Many gas block configurations have been produced during the continuing development cycle for M16 style firearms. While sharing an internal configuration intended for the primary function of conducting combustion gas from the firearm barrel gas port to a gas tube or operating element, prior art blocks differ externally. Gas block external configuration varies substantially depending on the intended block secondary functions.
Gas blocks in general can be categorized, however, as three basic styles: integral front sight, integral rail, and low profile. Prior art gas blocks are attached to the firearm barrel by various means, including pins, clamping, set screws or a combination of these means. Features of these prior art gas blocks, depicted in FIGS. 1-4, will be discussed in detail presently in order to demonstrate the advantages of the present invention.
Although functional, prior art gas blocks have several important shortcomings:                (a) As a group, after initial factory installation, prior art gas blocks can be characterized as difficult to remove and reinstall while maintaining alignment to the firearm gas port. At initial manufacture, the blocks are generally aligned with the barrel port by means of special fixtures. Consequently, when the gas blocks are removed for any reason, skilled armorers and/or fixtures, or a trial and error method must be used to ensure proper alignment at installation. Robbins, U.S. Pat. No. 5,945,626 includes a key, separate from the securing means, thus requiring two parts to accomplish aligning and securing the gas block, unlike one embodiment of the present invention.        (b) Free-float tubular handguard tubes are becoming the standard for many M16 style firearms. With the exception of some low profile gas blocks (which don't permit mounting a barrel-referenced front sight), prior art gas blocks must be removed from the firearm prior to installation or removal of the handguard. When the prior art gas block includes a rail, the rail as well as the gas block gas port must be re-aligned to the barrel and receiver of the firearm when the gas block is re-installed. This can be a difficult task in the field. Therefore, it is of substantial benefit to the user if the handguard can be removed/re-installed (for cleaning or repair, for example) without the need to remove the gas block.        
(c) Prior art integral rail gas blocks are configured to attach ancillary devices substantially outboard of the barrel, the gas block rail being located radially outward of the gas tube, thereby reducing firearm handling qualities and adding to firearm bulk.
It would be highly advantageous, therefore, to remedy these and other deficiencies embodied in the prior art. The advantages of this improved gas block will become apparent after the consideration of the ensuing description and drawings.